1. Field of the Invention
This invention relates to stylus input devices for computer data or information. More particularly, this invention relates to acoustic contact sensing means for said stylus device.
2. Description of Related Art
There are many computer input devices which provide the location of a stylus in a working area. Examples of these devices are data tablets, light pens, and laser scanning devices etc. (For example, see U.S. Pat. Nos. 3,134,099 and 4,564,928, Japanese Patent Nos. 55-39937 and 57-14978; and IBM Technical Disclosure Bulletin, Vol. 12, August 1969, pg. 390). These devices are frequently used to specify single points, for example, to define the ends of lines to be drawn in computer-aided design systems. However, although well suited for the above purposes, these devices are not suitable for handwritten input by use of the stylus.
Some of the principal requirements for acceptable handwritten computer input are a precision of 50-100 points per inch, a data rate exceeding 60 points per second, and a very sensitive mechanism to detect contact between the stylus and the working surface. The contact sensing feature is critical for segmenting the stream of coordinates into strokes typical of handwriting.
In most cases, location sensing techniques which provide adequate precision, detect the stylus at an ill-defined distance from the working surface. A common attempt at a solution to this inadequacy is to add an electrical switch which can be activated by pressing the stylus against the working surface. However, reliable switches require considerable force and/or displacement, making it difficult to write in a natural manner. Conversely, devices with good contact properties, such as surface acoustic wave sensors, do not have adequate resolution and/or data rates. In some cases the contact sensor would be activated in error if the hand touched the writing surface.
It is therefore an object of the present invention to provide an extremely sensitive, high resolution, contact sensing device for handwritten computer input.
It is a further object that said above contact sensing device be adaptable to any number of different coordinate input devices.